The present invention relates to a zinc oxide transparent electric conductor having zinc oxide as its primary component, a sputtering target for forming the foregoing transparent electric conductor, and a method of producing the foregoing target. The term “transparent electric conductor” as used herein includes a transparent electric conductive film.
Today, the material used most as a transparent electrode of flat panel displays and the like is ITO (Indium Tin Oxide), which is obtained by doping indium oxide (In2O3) with an appropriate amount of tin (Sn). The reason why ITO plays the leading part in a transparent electric conductor is that the various characteristics of ITO such as low resistivity and high transmittance in the visible light region, which are required in a transparent electric conductor, are superior in comparison to transparent electric conductors prepared from other materials.
Nevertheless, indium (In) as the raw material to be used in ITO entails problems in that the cost of the end product will increase because indium is expensive, and the supply of materials may be feared for resource depletion because indium is a scarce resource. Although the development of a zinc oxide transparent electric conductor having zinc oxide as its primary component is being actively developed for the substitute material of ITO, there is still a problem in that the resistivity is significantly high in comparison to ITO.
The reason for this is that the conventional development policy of zinc oxide transparent electric conductive materials was mostly based on the search of a single dopant. In other words, the foregoing policy is to find an element in a periodical table that will serve as an n-type dopant and emit electrons as a result of doping the zinc oxide as the parent material with such element. Specifically, for example, in most cases, a target doped with a candidate element having an atomic valence that is greater than bivalence, which is the valence of zinc, in an appropriate range of concentration is prepared, and this is subject to sputter-deposition to evaluate the resistivity of the film.
As a result of this development policy, various candidate dopants having an atomic valence of trivalence (refer to Patent Document 1) and tetravalence (refer to Patent Document 2) were discovered, and the most appropriate are gallium (Ga) and aluminum (Al). Even with a zinc oxide transparent electric conductor added with the foregoing elements, however, the actual condition is that the resistivity of such candidate dopants is far inferior to ITO.
Consequently, in recent years, attempts are being made for adding another type of element or compound in addition to the n-type dopant to zinc oxide. For example, Patent Document 3 describes that adding silver to zinc oxide and oxide of an element having trivalence is effective for achieving low resistivity of the transparent electric conductive film, but with this method the resistivity reduction effect is insufficient.
Moreover, Patent Document 4 describes a method of obtaining the intended etching speed by co-doping zinc oxide with an n-type dopant and cobalt (Co) or vanadium (V) and controlling the chemical properties, but it is described that the resistivity of the zinc oxide film obtained with this method increases according to the increase in concentration of the added cobalt or the like, and is not intended to reduce the resistivity.    [Patent Document 1] Japanese Patent Laid-Open Publication No. S61-205619    [Patent Document 2] Japanese Patent Laid-Open Publication No. S62-154411    [Patent Document 3] Japanese Patent Laid-Open Publication No. H9-320344    [Patent Document 4] Japanese Patent Laid-Open Publication No. 2002-75062